The present invention relates to a fluidized bed apparatus and a process for carbonizing wood and/or wood residues, especially in a fluidized bed apparatus. The invention also relates to a process and an apparatus for the production of activated carbon.
The timber industry generates considerable quantities of wood residues. Bark is produced in the debarking of the logs. Sawing of the debarked logs produces slabs, edgings and sawdust. If the solid slab and edgings are chipped for export pulp chips, the chips must be screened and this produces additional residues in the form of undersized and oversized woodchips. The timber is often seasoned then dressed and docked to length before despatching to the markets. This produces dockings and planer shavings residues. In many circumstances, there is poor utilization of the wood residues. Some of the bark can fetch a market as a garden mulch if it is sized. Sawdust, chip fines and planer shavings are used by the mill for energy, in most cases, as a fuel for kiln drying timber and, in a small number of mills, for electricity generation. However, there are generally many more residues produced than the energy requirements of the sawmilling industry. Sawdust is also used for composting into garden potting mixes. Generally, there is a large surplus of residues as the markets for horticulture are diminishing as demand for the dark P.radiata bark has fallen as the lighter wood fibre product has gained in popularity. The use of chipped prunings is also increasing as Councils and householders are turning to recycling and self-sufficiency. Where the residues are not utilised, they must be disposed of by incineration and land-fill dumping. The disposal of wood residues not only puts a severe cost on the sawmilling operation, particularly with the pollution restrictions imposed on air quality, water effluent into water-ways and ground water and the diminishing land-fill availability, but it represents a loss of a potentially valuable wood resource. Unlike coal which can be left in the ground to mine at a later date, wood residues cannot be stored on a long term basis and need to be processed when produced Wood residues are also bulky, as in the case of sawdust and shavings, and can also contain a considerable quantity of water. Processing or utilization in-situ offers the advantage of avoiding the high cost of transportation.
When wood is heated, it loses free and hygroscopic water after which it will carbonize at temperatures in excess of 270xc2x0 C. Gas and vapours are produced during carbonisation which, at some stage, becomes exothermic. There are many complex reactions occurring at the same time in the thermal decomposition of the various chemical components of wood. Practical carbonizing temperatures are in the range of 400-700xc2x0 C. in order to produce charcoal with low-volatile content without excessive shortening of equipment life.
The volatile products consist of combustible gases and vapors. The energy value of the volatile products represents some 50% of the gross calorific value of the original dry wood. Although there are significant proportions of valuable chemical compounds present in the volatile products, production on a larger scale is required to economically justify the fractionation and recovery of these compounds. However, the typical scale of operations in individual timber mills cannot produce economic quantities of volatile products. This material can present problems in handling due to its acidic, corrosive nature and it would be a serious pollutant if discharged into the environment. One way of handling the volatile products is to bum them as they are produced before they are able to condense. The waste beat can be recovered to supply the energy requirements of the industry, hence optimizing the thermal efficiency of the carbonization process.
A viable system for the sawmilling industry would perform the threefold purpose of disposal of the wood residues, supply the energy requirements of the milling and seasoning operations, and upgrade the excess material into a product which can provide a profitable return.
The applicant""s earlier Australian Patent No. 547130 described a process that achieved the above aims. This earlier patent described a process for carbonizing wood by feeding wood into a fluidized sand bed preheated to a temperature above the carbonizing temperature. The fluidized bed was fluidized with a gas mixture that included an oxidizing gas. The reaction conditions within the bed was selected such that all or a major proportion of the volatile components of the wood were burnt during carbonization, either as the volatiles were produced in the bed or partially burnt in the bed and the remainder in an afterburner. The burning of the volatile components provided sufficient energy to supply the heat required by the process as well as provide an excess of beat Charcoal produced by the process was recovered as product
The process described in Australian Patent No. 547130 provides a satisfactory process for treating timber milling residues to obtain a value-added product.
The present inventors have now developed an improved process for carbonizing wood, such as timber milling residues.
Activated carbon is an amorphous form of carbon having a very high specific surface area. Activated carbon has high absorptivity for a large number of substances and is widely used as an adsorbent in many industries, including water treatment, sugar refining, gold mining, brewing, gas adsorption and air conditioning, to name but a few. Activated carbon may be obtained by the destructive distillation of wood, nut shells, animal bones or other carbonaceous materials. It is also possible to produce activated carbon by activating a carbon feedstock, such as charcoal. Activation occurs by heating the material to be activated to an elevated temperature, such as 800-900xc2x0 C. with steam or carbon dioxide to produce a carbon material having high porosity and a specific surface area that may be in excess of 1000 m2/g.
According to a first aspect, the present invention provides a process for carbonizing wood residues to produce charcoal, said wood residues including wood or woody-type particles of varying size and/or moisture content, the process including the steps of feeding the wood residue to a fluidized bed having a plurality of wood residue inlets, the wood or woody-type particles of varying size and/or dryness being fed to differing ones of the plurality of wood residue inlets according to an expected time for carbonization for said wood or woody-type particles, the fluidized bed including a bed of inert particulate material fluidized with or having injected therein a gas or gas mixture containing an oxidizing gas, carbonizing the wood residues in the fluidized bed under reaction conditions selected such that volatile components in the wood residues are removed during carbonization and are burned in or above the bed or in an afterburner to thereby supply the heat requirements for carbonization and separating charcoal from the inert particulate material.
Preferably, the residence time of a wood or woody-type particles in the fluidized bed is largely determined by the wood residue inlet through which the wood or woody-type particle is fed to the bed. In this manner, different residence times in the bed for different particles may be obtained by feeding the particles through different wood residue inlets.
The process may preferably further include grading the wood or woody-type particles into a plurality of grades according to expected time for carbonization and feeding different grades to different of the plurality of wood residue inlets. The plurality of grades may include grades based upon particle size, moisture content, wood species and type of residue, for example, wood or bark. The wood or woody-type particles may include sawdust, planer shavings, shredded dockings, woodchips, bark, barkchips and larger wood such as blockwood.
According to a second aspect the present invention provides a process for production of activated carbon from a carbon feedstock including the steps of providing a fluidised bed reactor, adding particulate material to the fluidised bed reactor to form a bed of particulate material, adding the carbon feedstock to the fluidised bed reactor, fluidising the bed and activating the carbon feedstock to produce activated carbon and recovering activated carbon from the bed.
Preferably the carbon feedstock is the product of the first aspect of the present invention.
Preferably, the step of activating the carbon feedstock includes adding steam to the fluidised bed. More preferably, steam is used as the fluidising gas in the fluidised bed. The steam is most preferably superheated steam. Activating gases other than steam may be used. Another suitable activating gas is carbon dioxide.
According to a third aspect the present invention provides an apparatus for carbonizing wood residues to produce charcoal including a fluidized bed reactor having a plurality of wood residue inlets for supplying wood residues thereto, a discharge outlet for removing fluidized bed contents from the fluidized bed and at least one fluidizing gas inlet, characterized in that residence time of a wood or woody type particle in the fluidized bed reactor is dependent upon the inlet through which the wood or woody type particle is fed to the fluidized bed reactor.
The apparatus may further include separating means for separating charcoal from the fluidized bed contents removed from the discharge outlet and return means for returning inert particulate material to the fluidized bed reactor. The return means may comprise a conduit connected to an inlet. This inlet may be one or more of the wood residue inlets. The conduit may be provided with particulate material transport means, which may be a conveyor, a screw conveyor, an augur, pneumatic conveying means or gaseous conveying means.
The separating means may comprise a screen means, especially a vibratory screen means.
The fluidized bed reactor should also include an exhaust gas outlet for removing fluidizing gas and combustion gas from the fluidized bed reactor. A gas-solid separation means may be provided to separate elutriated solids from the exhaust gas. The gas-solid separation means may be a cyclone or an electrostatic precipitator.
The apparatus may further include an afterburner for burning any uncombusted sly volatiles in the exhaust gas. A pre-heater for heating the fluidized bed reactor may also be provided to pre-heat the fluidized bed at start-up or during low temperature operation.
The apparatus may also include heat recovery means for recovering heat from the fluidized bed in the reactor. Heat recovery means may also be provided for recovering heat from the exhaust gas and/or the afterburner.
According to a fourth aspect the present invention provides an apparatus for producing activated carbon including a furnace, a reactor positioned inside the furnace, the reactor including solids inlet means for supplying solids to the reactor and gas inlet means for supplying gas to the reactor, at least one pipe connected to the gas inlet means of the reactor, said at least one pipe having at least a portion of its length extending within the furnace whereby gas flowing through said at least one pipe to the reactor is heated to an elevated temperature by the furnace.
Preferably, the reactor is a fluidized bed reactor.
Preferably, that at least one pipe has a substantial portion of its length extending within the furnace. Preferably, that at least one pipe is positioned within the furnace and external to the reactor.
Preferably, the apparatus further includes a gas manifold inside the furnace, the gas manifold having an inlet and a plurality of outlets, each of the plurality of manifold outlets having respective ones of a plurality of pipes extending therefrom, the plurality of pipes being connected at their other ends to the gas inlet means of the reactor.
Preferably, the gas manifold is positioned external to the reactor. Preferably, the gas manifold extends substantially around the reactor. Preferably, the gas manifold is positioned at an upper part of the furnace.
Preferably, the gas inlet means of the reactor is a fluidizing gas inlet means of a fluidized bed reactor. Preferably, the gas inlet means of the reactor comprises a plurality of gas ports. The gas ports are connected to respective ones of the plurality of pipes.
The furnace may include at least one burner.
The reactor may include exhaust gas removal means for removing exhaust gas therefrom. The exhaust gas removal means may include solids removal means for removing solids from the exhaust gas. The solids removal means may comprise a screen, a filter or a cyclone. The solids removal means may return removed solids to the reactor.
The reactor may also include solids outlet means for removing solids from the reactor. However, the reactor may be configured, such that solids removal may take place via the solids inlet means. It may also be configured such that solids removal may take place by elutriation through the exhaust gas outlet.
In use of apparatus of the present invention is used to produce activated carbon, calcined alumina and feedstock carbon are supplied to the reactor. As the reactor is inside the furnace, the furnace heats the contents of the reactor to the desired activation temperature. Fluidizing gas, in this case steam, is supplied to the gas inlet of the reactor via at least one pipe extending within the furnace. As the steam flows through the at least one pipe it is superheated to the desired temperature by the furnace. As the steam enters the gas inlet of the reactor, it is at the desired temperature and activation of the carbon feedstock proceeds.
Although the apparatus of the present invention has been described as being used for production of activated carbon, it will be appreciated that the apparatus may be used in any process where it is desired to pass a gas at elevated temperature through a bed of solids material.
The inert particulate material may comprise any suitable particulate material that is able to be fluidized and does not undergo substantial reaction in the operating conditions experienced in the fluidized bed. Sand is a suitable particulate material for use in the present invention. Fine sand is preferred over coarse sand because fine sand gives better heat transfer to submerged surfaces (such as a heat exchanger within the bed) and to cold feed particles of wood residues and promotes better combustion of volatile gases within the bed. Fine sand also causes less attrition of submerged surfaces and products then coarse sand. Ilmenite sand has been found to be particularly suitable because it has a fine size (typically below 1 mm), is very dense, wear resistant and has a high melting point. Thus, it can be used over a wide range of operating conditions. Ilmenite sand is a naturally occurring, relatively abundant, low cost material.
Calcined alumina may also be used as a suitable particulate material in the present invention.
The reaction conditions used in the fluidized bed are those that are suitable for carbonizing wood residues. The temperature may range from 250xc2x0 C. to about 1000xc2x0 C., preferably 250xc2x0 C. to 700xc2x0 C., more preferably with a temperature range of 400-600xc2x0 C., most preferably at about 500xc2x0 C., being suitable for optimum charcoal production.
The present invention is based on the rapid drying and pyrolysis of residue particles in a fluidized bed which has a high rate of heat transfer between hot particulate material and wood particles. The volatile products of carbonization produced within the fluidized bed are combusted on encountering oxygen. The volatiles burn rapidly, producing heat while the charcoal formed burns more slowly. The charcoal product is recovered from the fluidized bed by separation and cooling before it has time to bum to a significant extent.
By maintaining an excess of oxygen in the bed, charcoal production is controlled by the kinetics of the combustion of the charcoal and volatiles. At lower bed temperatures, typically 400xc2x0 C.-500xc2x0 C., the rate of combustion of charcoal is slow and recovery of charcoal can be high. At high temperatures above 600xc2x0 C., charcoal can burn rapidly and result in complete combustion. If bed temperatures are too low, such as 400xc2x0 C. or lower, the rate of combustion of volatiles can become too slow to maintain the heating of the bed and much of the volatiles can escape the bed without burning. When this occurs, supplementary preheating of the bed may be necessary and if the preheat burners can not burn the volatiles escaping from the bed, an afterburner may also be required to eliminate the discharge of unburnt volatiles.
Heat can be recovered from the combusted off-gases by using a heat exchanger. It is also possible to recover heat from the bed with a heat exchanger submerged in the fluid bed. This has the advantage of utilizing the high heat transfer coefficients within the bed. In addition, by removing the heat from the bed, it is possible to increase the carbonizing capacity of the bed.
Another feature of the present invention is that the operating conditions can be altered to meet varying requirements for either charcoal production or heat production only. This is an especially useful feature where the carbonization process forms part of an integrated saw mill. Such integrated saw mills typically have drying kilns associated therewith for drying sawn timber.
It has been found that by operating a fluid bed fluidized with air at bed temperatures above 650xc2x0 C., rapid combustion occurs for sawdust and woodchips. Efficient, total combustion occurs above 700xc2x0 C. Maximum heat recovery can be derived from this system through heat exchangers within the bed and/or in the combustion off-gases. If the fluid bed temperature was reduced, the charcoal formed as an intermediate phase burns more slowly and can be recovered as a byproduct. It was found that at a bed temperature around 500xc2x0 C., good recovery of charcoal can be achieved with complete combustion of volatile products of carbonization within the fluidized bed. (Carbonization, pyrolysis, or thermal decomposition occurs when wood is heated above 250xc2x0 C. The hemicellulose component of wood decomposes at the low temperature to produce a charred wood). The bed temperature can be regulated by controlling the feedrate of wood fuel. Within the design capacity of the system, a high feedrate of fuel will increase combustion temperature within the fluid bed and vice versa.
This system can be used as a boiler or process heat production unit with a high turndown ratio exceeding that of conventional combustion systems by lowering the combustion temperature to a theoretical limit of 250xc2x0 C. to produce a byproduct charred wood In practice however, the bed temperature should be around 500xc2x0 C. but can be lowered if the volatile gases which escapes the bed is burnt in an afterburner, or secondary combustion chamber and enough of this heat transferred to the fluidized bed to sustain the carbonization temperature. At 500xc2x0 C., the charcoal product has a higher fixed carbon content than low temperature chars.
The system becomes a very flexible system for sawmills which can use the process heat for kiln drying of its sawn products. During the heatup cycle of the kiln and charge, the heat demand is at its maximum. Under this situation, the fluid bed unit can produce its maximum heat output by operating at a high bed temperature for complete combustion and the maximum design feedrate of wood fuel. As the timber charge is kiln dried at mid cycle, the heat demand drops to a low level. Here, the fluid bed can operate in its charcoal producing mode to reduce heat output and maximize charcoal production. Wood residue production in typical sawmills far exceeds its is energy requirements. In energy terms, only 20% of the total residue production can provide all the energy requirements. Some of this excess energy can be converted into a saleable charcoal byproduct. The system can therefore be tailored to process the residue production and provide the energy needs to sawmills.
Charcoal is one of the products of the present invention.
Product charcoal is separated from the inert particulate material in the fluidized bed and recovered. Product charcoal may be separated from the inert particulate material in two ways:
i) at least a portion of the contents of the fluidized bed are removed from the bed and the charcoal separated from the inert particulate material, for example, by sieving or screening. The charcoal is recovered as product and the inert particulate material is recycled back to the fluidized bed, preferably whilst still hot. Preferably, at least a portion of bed contents is continuously removed and the separated inert particulate material is continuously recycled to the bed. This method of separation is limited by the particle size of inert particulate material as the screen size must be larger than the particle size of the sand.
ii) charcoal fines may be separated and recovered by elutriation from the bed. The upward velocity of the fluidizing gas and combustion gases will lift out particles of a size that their terminal velocity is less than the upward velocity of the gases. The wood-derived charcoal particles have a low particle density. To maximize the degree of charcoal fines elutriation, the inert particulate material preferably has a high particle density. Furthermore, fluidization velocities are preferably below a velocity that would carry off particles of the inert particle material. With bed attrition, breakdown of the inert particulate material could see the fines caused by such breakdown reporting in the charcoal fines product.
It will be appreciated that the present invention also extends to cover any other suitable process for recovering charcoal from the fluidized bed.
The fluidizing gas used in the present invention is preferably air. The supply of fluidizing air should preferably be such to ensure adequate excess of oxygen for the combustion of volatiles within the bed. The maximum flowrate that may be used is that which can just promote the mixing of the inert particulate material and the wood residues. Operating below this regime may result in bed segregation occurring.
The fluidized bed is preferably able to be sectioned off so that the superficial velocity can be adjusted to optimize each section. By turning off fluidizing gas to a particular section, the bed in that section will be slumped or immobilized.
The fluidized bed is preferably configured as a linear slide fluidized bed with multiple feed injection points along the bed. In this arrangement, the positioning of the multiple feed injection points corresponds to the required residence times to carbonize the grade of wood residue passing through each of the wood residue inlets or injection points. To make the unit shorter, a unit with internal walls in the fluidized bed section to channel the material in a spiral fashion from inlet to outlet may be used
If required to promote better mixing of wood residues and the inert particulate material, one or more mechanical stirrers may be introduced.
The fluidized bed may have a single discharge point. The fluidized bed may be provided with a recycled particulate material inlet for returning recycled inert particulate material to the bed. Alternatively, the recycled inert particulate material may be returned to the bed via one or more of the wood residue inlets.
In the second aspect of the present invention the carbon feedstock may be charcoal, such as charcoal produced from saw milling and other wood residues such as bark, wood chips and saw dust. The charcoal is suitably produced by the process described in the applicant""s Australian Patent No. 547,130. However, especially preferred is the charcoal as produced according to the process of the present invention. Other carbon feedstocks may also be used. Other suitable carbon feedstocks include coke and chars based on various carbonaceous sources such as biomass, nut shell, fruit seed, fruit kernel, animal, peat, brown coal, coal, anthracite, petroleum, natural gas or other organic compounds and substances. Straight carbonaceous materials of the abovementioned feedstocks can also be used as the process could convert these to an intermediate char stage.
The temperature during the activation step is preferably within the range of 650-1000xc2x0 C., more preferably within the range of 800-900xc2x0 C., most preferably within the range of 800-850xc2x0 C.
The time required to complete the activation step preferably falls within the range of 10 minutes to 3 hours.
The particulate material is preferably calcined alumina although any other particulate material that is stable at the temperatures encountered during the process and does not deleteriously react with the feedstock carbon or the activated carbon may at also be used. Sand is an example of another suitable particulate material that may be used in the process. The particulate material ideally should not break up or wear down during fluidisation not melt nor soften under the high temperature process conditions.
A particle size analysis of a calcined alumina suitable for use in the present invention is as follows:
In cases where steam is used as the activating gas, one of the by products of the process is water gas. In such cases, the process preferably further comprises recovering the water gas and using the water gas to provide the heating or energy requirements of the activating process. If there is a surplus of water gas, the surplus gas can be reticulated as a fuel gas and/or used to generate electricity that may be utilised on-site or sold to the electricity supply grid.
Waste heat from the process may be used to generate steam to supply the process steam requirements.
The process of the present invention may be a continuous process or a batch process. If the process is to be used to produce activated carbon from charcoal obtained from wood residues, the process is preferably operated at the wood processing site (such as a saw mill or a wood chipping plant). As the size and quality of the carbon feedstock may vary widely in this situation, a batch process is preferred in order to provide greater flexibility during operation. For example, residence times are much easier to alter in a batch process because it is simply a matter of changing the batch time.
If steam activation is used and the by-product water gas is recovered for energy requirements, it is preferred that a plurality of fluidized bed reactors are used in order to even out the production of water gas and to ensure that water gas is constantly available. For example, if two fluidized bed reactors are used, one may be activating charcoal and producing water gas whilst the other is being emptied and re-charged.
As mentioned above, calcined alumina may be used as the particulate material in the bed. Calcined alumina has a relatively low density and small particle size and this enables a deeper bed to be used for a given pressure drop, when compared to denser and coarser bed materials such as sand. Alternatively, a lower bed weight can be obtained which allows a lower strength reactor to be used. This may be an important factor to consider because the reactor is operated at elevated temperatures. Moreover, calcined alumina is white in colour and provides a visual contrast to the black-coloured activated carbon. This allows a visual inspection to determine if adequate separation of the product from the particulate material is being obtained.
The product activated carbon may be recovered by separating it from the particulate material by any suitable method known to the person skilled in the art The activated carbon is suitably removed from the solid material by sieving. In a continuous process, the activated carbon may be recovered by periodically removing a portion of the solid material from the bed and separating the actuated carbon therefrom. Alternatively, the activated carbon may be elutriated by the exhaust gas and recovered therefrom. In batch processes, the entire solids load of the batch may be removed from the fluidized bed reactor and the activated carbon subsequently removed therefrom. The hot bed material can be refused for the next carbon activation batch in the hot state to conserve heat energy.
The present invention also relates to an apparatus for producing activated carbon.
A preferred embodiment of the present invention will now be described with reference to FIG. 1. It will be appreciated that the following description is intended to be illustrative of an embodiment the present invention and should not be considered to limit the present invention to the embodiment described.